Interaction between fragments of C3 and lymphocytes is currently under investigation. Of the four major fragments of C3 (C3a, C3b, C3c and C3d), only C3b activates B cells to elaborate a chemotactic factor for mononuclear cells in the absence of proliferation. This chemotactic lymphokine appears identical to that released by T and B cells when stimulated with appropriate antigen(s) or mitogen(s). Studies currently in progress are directed toward elucidating; 1. Mechanism by which C3b activates lymphocytes. 2) Role of C3 in B cell response to T-dependent antigens. 3) Modulatory role of C3d in B cell activation by cell surface signals (Fc, C3b, Ig).